The present invention relates to surface sizing and coating apparatus for a paper web of a paper machine.
Known size presses comprise a coating pool through which a sizing agent, or other coating agent, is applied to the web. However, the web "wets" in the coating pool and its strength decreases. Furthermore, the web is without support and subjected to great hydrodynamic forces in the pool. This results in an increased risk of web breakage. It is known to use extruding apparatus or equivalent devices to supply the coating agent in connection with the coating roller. The coating agent supplied by such apparatus is leveled by knife or doctor coating apparatus. However, no pressing nips have been used in connection with such apparatus and the penetration of the coating agent into the web has remained poor.
The surface sizing and coating apparatus of the invention supports the web to be coated well during the application of the coating agent, which application is controlled. Thus, the speed of the web may be essentially increased, compared to previous speeds.
The surface sizing and/or coating apparatus of the invention for a paper web, or the like, comprises a plurality of coating rollers which form with each other one or more coating nips through which the web to be coated is arranged to pass.
The surface sizing of the paper web is usually made in an "on-machine" size press of the paper machine. The size press undertakes several different operations affecting the properties of paper and board.
As previously known, the paper web is run through the roller nip of the size press. The sizing agent is applied to the paper in the press by forming a pool or pools in front of the nip. The web gets wet in the pool and adsorbs sizing agent. The sizing agent is pressed into the web, due to hydrodynamic pressure, between the rollers of the size press.
As previously known, the oldest size press is the so-called vertical size press, in which the rollers are placed one on top of another and the paper runs horizontally between said rollers. In the known horizontal press, the rollers are in the same horizontal plane and the paper runs vertically between said rollers. The third known size press is the so-called oblique press, in which the rollers are placed one on top of another at an angle of about 45.degree..
The diameters of the rollers of size presses are important regarding the suitability of operation. In the known size presses, the diameters of nip rollers are fixed at about 800-1500 mm at operating speeds of 10 to 17 m/s. In known apparatus, it is attempted, by increasing the diameter of the rollers, to prevent splashing in the nip entrance pool, because this disturbs the even wetting of the paper web.
As previously known, rubber is most often used as the covering material of the rollers of size presses. Also known are presses in which the hard roller is metal and the soft roller is rubber. Thus, softer and harder pressing zones are used. Rollers with greater diameters are used at higher operating speeds. A good coating agent penetration is achieved not only by a sufficient wetting time, but also by a high pressing load of 40 to 50 kN/m.
The production of fine paper is shifting to large paper machines, which imposes great requirements on size presses. A poorly functioning size press decreases the productivity of the whole paper machine. The surface sizing and coating apparatus of the invention thus provides solutions which have been developed especially regarding high operating speeds.
Although the term "size press" is used herein, it must be understood that in this context the term "sizing agent" is used in a broad meaning, so that it also encompasses other known treatment agents for paper or board webs which are suitable for use in apparatus similar to that of the invention.
The size press of the invention is also suitable for use as a so-called "off-machine" coating machine.
A serious disadvantage of known size presses and similar coating machines is the splashing of the sizing agent or similar coating agent, which may even entirely spoil the web to be produced. The disadvantages caused by splashing are accentuated when the speeds of the size presses are increased. Splashing problems are thus especially accentuated in "on-machine" size presses.
A disadvantage of previously known coating apparatus based on the use of a coating pool is that the coating agent splashes from the pool to the surroundings, and even onto the web. Another disadvantage is that the web gets softer when passing through the pool, resulting in a decrease of the strength of the web. The risk of web breakages is then increased, because the weak web is subjected to large hydrodynamic forces when passing through the pool, and the web is not supported in the pool.
In order to avoid the aforementioned disadvantages, the apparatus of the invention starts from the previously known basis of applying the coating agent onto the web by known knife or doctor coating methods, and supplying the coating agent by known extruder apparatus, applicator rollers, or equivalent means. Of interest is an article entitled, "Bestrykning forbattrar tryck-papperets yta" by Ingrid Fineman in Kemisk Tidskrift 1979, No. 13, for example. This article, among others, presents various means for supplying the coating agent, and knife and doctor coating devices connected with them. However, pressing nips are not used in the known apparatus. It is well known that it is only possible to apply the coating agent to the surface zone of the web by knife or doctor coating, whereas the coating agent does not penetrate into the fiber structure of the web to a substantial extent. This partly explains the fact that in knife or doctor coating, the strength of the web does not essentially decrease due to the coating agent. However, in many cases it is very desirable that the coating agent penetrate into the fiber structure of the web, as deep as possible, so that the web becomes, among other things, as strong as possible.